Ethernet CSMA/CD, Framing, SNAP, Repeater, Hub, 10Mbit/s Technology Agenda Introduction CSMA/CD Elements and Basic Media-Types Repeater, Link Segments Framing Ethernet, v4.7 2 Page 21-1
Origin of IEEE 802.3 (Ethernet) bus topology based on coax-cables passive, uninterrupted coupling shared media like the Ether of air bidirectional signal-propagation termination resistors avoid signal reflections definite transmitting power of network stations limits cable length and number of (receiver-) stations two types with baseband transmission with Manchester encoding, 10 Mbit/s 10Base5 "Yellow Cable 10Base2 "Cheapernet" one type with modulation (broadband) 10Broad36 (broadband) Ethernet, v4.7 3 Basic Idea of Ethernet Bus System shared media used in half duplex mode passive coupling bidirectional signal-propagation terminating resistor simulating an infinite line Ethernet, v4.7 4 Page 21-2
Agenda Introduction CSMA/CD Elements and Basic Media-Types Repeater, Link Segments Framing Ethernet, v4.7 5 Media Access Control of Ethernet 1 CSMA/CD Carrier Sense Multiple Access / Collision Detection access control based on contention network stations listen to the bus before they start a transmission network stations can detect ongoing transmission (CS) and will not start own transmission before ongoing transmission is over but still simultaneous transmissions (MA) cause collisions (bus conflict) collisions are detected (CD) by observing the DC-level on the medium Ethernet, v4.7 6 Page 21-3
Media Access Control of Ethernet 2 conflict resolution aborting of transmission by all involved stations sending of a JAM-signal (32 bit) to make sure that every station can recognize the collision collision is spread to a minimum length starting a random number generator to create a timeout value truncated binary exponential backoff algorithm (the more often a collision occurs the larger is the range for the random number) after timeout expired, station attempts a retransmission number of retransmission-trials is limited to 16 after 16 collisions in a sequence a error is signalled to the higher layer Ethernet, v4.7 7 Collision Window / Slot Time 1 A B t = x... A starts transmission A B t = x + dt... B starts transmission Ethernet, v4.7 8 Page 21-4
Collision Window / Slot Time 2 Collision!! A B t = x+ tp... B detects collision tp... signal propagation time from A to B A B t = x + 2tp... A detects collision Ethernet, v4.7 9 Collision Window / Slot Time 3 worst case stations have to wait (have to send bits) twice the maximum signal propagation time for reliable collision detection otherwise a collision may not be seen by the transmitting station the maximum allowed time for that in Ethernet transmission system is called collision window or slot time 10 Mbit/s Ethernet defines 51,2 microsecond for the collision window / slot time 10 Mbit/s means 1 bit every 100ns therefore 51,2 microsecond is equal to 512 bits hence the minimal frame length is 64 byte Ethernet, v4.7 10 Page 21-5
Collision Window / Slot Time 4 there is an interdependence maximum propagation time (cable and electronic components) or slot time, data rate, cable length and minimum frame size if you choose one parameter, the others will follow the request for reliable collision detection during sending of a frame and the definition of a given Ethernet slot-time limits the physical distance (network diameter) of Ethernet LANs for 10 Mbit/s around 2500-3000 meters the request for fairness limits the maximum frame size, too 1518 byte is the maximum allowed frame size Ethernet, v4.7 11 Collision Window / Slot Time 5 Technology Bit-Time (sec) Collision Window (sec) Slot Time (bit-times) Minimum Frame (bit-times / byte) Distance (m) 10Mbit/s 100ns 51,2μs 512 = 512 / 64 2000-3000 100Mbit/s 10ns 5,12μs 512 = 512 / 64 ~200 1000Mbit/s 1ns 0,512μs 512 512 / 64 ~10-20 1000Mbit/s 1ns 4,096μs 4096* = 512 / 64 200 * by the usage of carrier extension / frame bursting Ethernet, v4.7 12 Page 21-6
Exponential Backoff Details Provides maximal utilization of bandwidth After collision, set basic delay = slot time Total delay = basic delay * random 0 <= random < 2 k k = min (number of transmission attempts, 10) After 16 successive collisions Frame is discarded, error message to higher layer and next frame is processed, if any Truncated Backoff (k<=10) 1024 potential "slots" for a station Thus maximum 1024 stations allowed on half-duplex Ethernet Ethernet, v4.7 13 Collision Detection Details 10Base2, 10Base5 Manchester with 40 ma DC level "high" = 0 ma, "low" = 80 ma Two signals at same time: DC Level < -40 ma 10BaseT Manchester with no DC offset Collisions are detected by Hub component which sends a "Jam" signal back in case two or more stations start at the same time Similar at 100BaseT and 1000BaseT Ethernet, v4.7 14 Page 21-7
Agenda Introduction CSMA/CD Elements and Basic Media-Types Repeater, Link Segments Framing Ethernet, v4.7 15 Physical Layer Functions parallel (Ethernet Controller) Manchester encoding/decoding serial (transceiver cable) transceiver cable Logical Link Control LLC Media Access Control MAC PLS AUI PMA MDI Medium 1-10 Mbit/s Data Link Layer Physical Layer AUI...Attachment Unit Interface, PLS...Physical Layer Signalling, MDI...Medium Dependent Interface, Interface, PMA...Physical Medium Attachment, MAU...Medium Attachment Unit Ethernet, v4.7 16 Page 21-8
PHY Sublayers Physical Layer Signaling (PLS) serves as abstraction layer between MAC and PHY PLS provides data encoding/decoding (Manchester) translation between MAC and PHY Using PLS service primitives Attachment Unit Interface (AUI) to connect with PMA Physical Medium Attachment (PMA) interface between PLS and MDI Medium Attachment Unit (MDI) specification of the various connectors Ethernet, v4.7 17 Media-Connection by Transceiver transmitter / receiver transceiver provides electronic circuits for: inserting and receiving signal currents collision detection measurement of DC level 10Base5: Level High (1) = 0 ma, Level Low (0) = -80 ma DC of Manchester-encoded signal = -40 ma two signals at same time: DC Level < -40 ma heartbeat function SQE Signal Quality Error jabber control jabber: continuously emitting of frames beyond the maximal frame size Ethernet, v4.7 18 Page 21-9
External / Internal Transceiver transceiver types: 10Base5, 10Base2, FOIRL (Fiber Optic Inter Repeater Link) and 10BaseT, 10BaseF (these types will be handled later in this presentation) external transceiver: AUI interface (with or without transceiver cable) connects end system and transceiver transceiver powered by end system integrated transceiver: transceiver is integrated on network card of end system network card provides necessary physical connector BNC (10Base2) RJ45 (10BaseT) ST (10BaseF) Ethernet, v4.7 19 AUI-Connection with 10Base5 Transceiver Computer (network driver plus LLC) Ethernet-card protocol firmware (buffer and DMA) Ethernet Controller Chip (MAC) transceiver cable (serial transmission) MAU Yellow (Thick) Cable transceiver CD JC tmt/rcv AUI tap MAU... Media Access Unit AUI... Attachment Unit Interface (15 pole DB9 connector) CD... Collision Detecting circuits JC... Jabber Control circuits tmt/rcv... transmit/receive circuits transceiver cable: 8 twisted pair lines for tmt+/-, rcv+/-, control +/- collision presence +/-, 3 lines for power, earth, shield Ethernet, v4.7 20 Page 21-10
10Base5 Parameter termination resistor 50 Ohm, 1 Watt transceiver (drop) cable max. 50m vampire transceiver Yellow cable min. 2,5m max. 500m maximal number of stations: 100 attachable only at marked points cable splitting via coax couplers individual cable parts have a length of 23,4m or 70,2m or 117,5m (wave minimum on standing waves due to inhomogeneous media) smallest bending radius: 254mm Ethernet, v4.7 21 Integrated Transceiver for 10Base2 computer Ethernet-card protocol firmware Ethernet controller transceiver CD JC tmt/rcv BNC coax connector Cheapernet (Thin) Cable BNC T-connector Ethernet, v4.7 22 Page 21-11
10Base2 Parameter termination resistor 50 Ohm > 0,5 Watt T-connector RG58 Cable min. 0,5m max. 185m maximal number of stations: 30 attachable at any points smallest bending radius: 50 mm Ethernet, v4.7 23 Agenda Introduction CSMA/CD Elements and Basic Media-Types Repeater, Link Segments Framing Ethernet, v4.7 24 Page 21-12
Repeater Repeater is an amplifier expanding the maximal distance of an Ethernet-LAN segment regenerate signals on the receiving port, amplify them, and send these signals to all connected net segments no buffering, just a short delay, which must be taken into account for the collision window collisions are detected and all other ports are notified by jam-signal optionally auto partition on erroneous ports collision domain is preserved by repeaters local repeaters directly connect two (coax) segments remote repeaters are connected by so called link segments Ethernet, v4.7 25 Local / Remote Repeater repeater set coax segment local repeater coax segment repeater set link segment repeater set coax segment remote Repeater coax segment Ethernet, v4.7 26 Page 21-13
Multiport Repeater - One Collision Domain repeater set coax segment repeater set link segment coax segment link segment repeater set coax segment coax segment coax segment Ethernet, v4.7 27 Link Segments for Repeater Interconnection link segment first implementation for repeater interconnection only point-to point connection only two devices are connected by a physical cable several types were defined fibre based copper based FOIRL (Fibre Optic Inter Repeater Link) maximal length 1000m first FO specification repeater - repeater Ethernet, v4.7 28 Page 21-14
Link Segments for Repeater Interconnection types cont. 10BaseFL (Fibre) asynchronous maximal length 2000m repeater - repeater, end system - multiport repeater 10BaseFB (Fibre) synchronous (idle signals during communication pauses) maximal length 2000m for repeater - repeater links only developed to overcome limitation based on repeater rules by defining a repeater less backbone infrastructure 10BaseFP (Fibre) passive hub, no active repeater function (remark: active means electrically powered) Ethernet, v4.7 29 Repeater-Rules 1 collision domain of an Ethernet LAN is limited collision window of 51,2 microsecond topology of repeaters must obey maximal 5 segments over 4 repeater-sets are allowed, in this case 2 segments have to be link-segments (rest arbitrary), length of fibre optic link segments must not exceed 500m each -> results in a maximum diameter of 2500m on 4 segments with 3 repeater-sets, the length of a fibre optic link segment must not exceed 1000m, the segments may be mixed in any desired way -> results in a maximum diameter of 3000m Ethernet, v4.7 30 Page 21-15
Repeater-Rules 2 R 500m 10 Base FL R R 500m 10 Base FL R 10 Base 5 10 Base 5 10 Base 5 500m 500m 500m 1000m R R R 10 Base FL 1000m 10 Base FL 10 Base 5 10 Base 5 500m 500m Ethernet, v4.7 31 Link Segments for End Systems 1 link segment was later also defined for connection of a network station (end system) to a multiport repeater using a dedicated point-to-point line reason for that: Ethernet was originally based on coax cabling and bus topology later an international standard for structured cabling of buildings was defined star wired to a central point(s) based on twisted pair cabling that excellently fits to Token ring cabling Ethernet had been adapted to that in order to survive Ethernet, v4.7 32 Page 21-16
Structured Cabling (LAN) Physical Wiring Should follow the principle of structured cabling Primary End system to first Hub (Repeater or nowadays a L2 Switch) Stockwerkverteiler CU-UTP, Category 5e or better FO for extreme conditions only Secondary Hubs to central functions Gebäudeverteiler FO-MM (FO-SM) Tertiary Interconnections of buildings FO-MM (FO-SM) Ethernet, v4.7 33 Structured Cabling (LAN) FO-SM (FO-MM) (2000m-100km) Building 2 GV GV Building 1 FO-MM (FO-SM) (500m-2000m) Hub, SV Hub, SV Hub, SV Hub, SV CU Cat 5e (100m) CU Cat 5e (100m) RJ45 outlet Ethernet, v4.7 34 Page 21-17
Link Segments for End Systems 2 link segment (cont.) 10BaseT (unshielded twisted pair) maximal length 100m 2 lines Tmt+-, 2 lines Rcv+-, RJ45 connector Manchester-Code with no DC offset collisions are detected by hub, if two or more signals are received at the same time, hub produce Jam signal on all ports, hence collision is recognized if signals are on the tmt and rcv line at the same time during transmission pause Start of Idle signal followed by periodic link test pulses (LTP) to check the link state every 16ms a 100ns lasting LTP is sent by LAN devices, no signal on the wire means disconnected repeater - repeater, end system - multiport repeater, end system - end system via cross-over cable Ethernet, v4.7 35 Link Segments for End Systems 3 link segment (cont.) 10BaseFL (Fibre) asynchronous, maximal length 2000m repeater - repeater, end system - multiport repeater repeater with more than two segments and different physics multiport repeater end-systems and multiport repeater in a star like topology repeater is called a Hub be careful using this expression because also used for L2 Ethernet-Switch main usage for 10BaseT in today's Ethernet networks Ethernet, v4.7 36 Page 21-18
Multiport - Repeater 10 Base T 10 Base FL repeater 10 Base FL repeater 10 Base 5 10 Base 2 Ethernet, v4.7 37 Multiport Repeater as Hub max 100m 10 Base T 10 Base T repeater 10 Base FL max 2000m repeater 10 Base T max 100m 10 Base T Ethernet, v4.7 38 Page 21-19
Agenda Introduction CSMA/CD Elements and Basic Media-Types Repeater, Link Segments Framing Ethernet, v4.7 39 IEEE 802.3 Frame Format preamble DA SA length LLC data FCS preamble... DA... SA... length... data... FCS... parameters: for clock synchronization (64 bit) (62 bits 10101...01010 + 2bits 11 as SD, bit synchronization within 18 bit times) Destination MAC-address (48 bit) Source MAC-address (48 bit) of IEEE 802.3 frame (16 bit) = octets following without CRC (46-1500) payload Frame Check Sequence (32 Bit) interframe gap 9.6 microsecond jam size 32 bit slot time 512 bits, minimal frame length 64 Byte (6+6+2+46+4, FCS counted, preamble not counted) -> at maximum 14880 frames per second maximal frame length 1518 byte (6+6+2+1500+4) Ethernet, v4.7 40 Page 21-20
IEEE-MAC-Address Format I/G U/L b45,...,b44........., b1, b0 destination address (48 bit) X U/L b45,...,b44........., b1, b0 source address (48 bit) I/G Individual / Group (only for DA): I/G = 0 individual address, I/G = 1 group broadcast (broadcast for a group is called multicast) address with all bits set to 1... broadcast-address hex FFFF FFFF FFFF (note: U/L is set to 1) U/L Universal / Local: U/L = 0 global address, administered by IEEE U/L = 1 local administered address bit 47 (x) not used for source address Ethernet, v4.7 41 IEEE Administered Addresses (U/L = 0) 0 byte 0 byte 1 byte 2 byte 3 byte 4 byte 5 vendor code (Organizational Unique Identifier OUI) serial number IEEE assigns each vendor of network components an unique vendor code (OUI, byte 0, 1, 2) vendors use byte 3, 4 and 5 for numbering their network components (serial number) called Burned In Address (BIA) Ethernet, v4.7 42 Page 21-21
Storage Format of 802.3 MAC-Address basic rule: I/G bit must be the first bit on the medium, so the transmitted address must have the following format: I/G U/L b45,..., b40 b39,..., b32... b15,..., b8 b7,..., b1, b0 0 1 00 1000 0000 0000... 0000 0000 1100 0001 802.3 sends the least significant bit of each octet at first so 802.3 must store each octet in memory in reverse order: also called Canonical Format b40,..., b45 U/L I/G b32,..., b39... b8,..., b15 b0, b1,..., b7 0001 00 1 0 0000 0000... 0000 0000 1000 0011 Ethernet, v4.7 43 IEEE 802.3 <-> Ethernet Version 2 IEEE 802.3 relies on LLC (802.2) and SAPs the protocol-type is indicated by SSAP and DSAP (LLC) Ethernet Version 2 uses a protocol-type-field instead of the length field there is no need for an additional sub layer (like LLC) in order to implement connectionless services only layer 3 is directly attached on Ethernet V2 some values for the protocol-type-field (Ethertype): 0x0800 IP, 0x806 ARP, 0x8035 RARP, 0x814C SNMP 0x6001/2 DEC MOP, 0x6004 DEC LAT, 0x6007 DEC LAVC, 0x8038 DEC Spanning Tree 0x8138 Novell Ethernet, v4.7 44 Page 21-22
Ethernet Version 2 (DEC, Intel, Xerox -> DIX) preamble DA SA type data FCS preamble... for clock synchronization DA...Destination Address (48 Bit) SA...Source Address (48 Bit) type...protocol-type field (16 Bit) (Ethernet Version II frame) Data... payload FCS... Frame Check Sequence (32 Bit) Ethernet V2 and 802.3 can coexist on the same cable, but each associated sending and receiving station must use the same format. Fortunately all type-field values are larger than 1518 (max frame length), so any incoming frame can be recognized and handled properly. Ethernet, v4.7 45 Ethernet Type over LLC with SNAP SNAP (Subnetwork Access Protocol) convergence protocol to transport Ethernet V2 type information over IEEE LANs reason: LLC SAP-fields (length 8 bit) can not carry some already defined Ethernet V2 protocol types (length 16 bit) note: some IEEE LANs require the usage of LLC e.g. Token Ring, FDDI DSAP SSAP control L3 - L7 802.x MAC 0xAA 0xAA 0x03(UI) SNAP data 802.x MAC Organizational Code Ethernet Type 3 octets 2 octets Ethernet, v4.7 46 Page 21-23
SAP, SNAP, Ethernet V2 - SUMMARY 802.3 with 802.2 (SAP) preamble DA SA length DSAP SSAP Ctrl data FCS 46-1500 layer 2 (LLC) Ethernet Version 2 ("Ethernet II") preamble DA SA type data FCS > 1518 org. code type 802.3 with 802.2 (SNAP) preamble DA SA length AA AA 03 SNAP data FCS layer 2 (LLC) Ethernet, v4.7 47 Page 21-24